Amorphous ferrosilicide film exhibiting semiconductor characteristics and method of for producing the same

ABSTRACT

Disclosed is an amorphous iron-silicide film fully exhibiting semiconductor characteristics close to those of β-FeSi 2 , which has not been able to be achieved through a conventional cluster ion beam deposition process, molecular beam epitaxial growth process, ion implantation process or RF-magnetron sputtering process. FeSi 2  is grown as a non-granular flat or continuous film on a substrate maintained at a temperature of less than 400° C. through a sputtering process under an Ar gas pressure of 5 mTorr or less using a FeSi 2  alloy target having a Fe:Si atomic ratio of 1:2, to obtain an amorphous FeSi 2  film exhibiting semiconductor characteristics. In particular, a facing-targets type is preferable as the sputtering process.

TECHNICAL FIELD

The present invention relates to an amorphous iron-silicide film exhibiting semiconductor characteristics and a method of preparing the amorphous iron-silicide film.

BACKGROUND ART

β-FeSi₂ is a direct transition semiconductor having a bandgap of 0.85 eV, and expected to be applied to a solar cell element and a light-receiving element for communication systems. The present inventor previously developed a method of depositing a FeSi₂ thin film which is in the β-phase in the state after it is just deposited on a substrate through a laser abrasion process (Patent Publication 1: Japanese Patent laid-Open Publication No. 2000-178713).

DISCLOSURE OF INVENTION

Japanese Patent Publication No. 01-31453 discloses that a ζ-FeSi₂ phase amorphous film to be formed as a stable solid solution when Si is in the range of 69 to 72.5 at. %. This amorphous film is prepared through a cluster ion beam deposition process in which Fe and Si to be deposited are injected from separate closed crucibles, respectively. The ζ-FeSi₂ phase amorphous film, for example, containing 68 at. % of Si exhibits an electrical conductivity of ˜1 Ω⁻¹ cm⁻¹ and a bandgap of 1.258 eV, which are not close to the characteristics of β-FeSi₂.

In order to obtain a high-quality film with an amorphous structure, it is required to allow particles to be supplied onto a substrate in a possibly high-energy state and in its atomic form, and then quenched by the low-temperature substrate which is not heated or cooled. However, even in a conventional RF-magnetron sputtering process which is regarded as an optimal process for obtaining an amorphous film in comparison with other conventional methods, such as a cluster ion beam deposition process, molecular beam epitaxial growth process and ion implantation process, it is difficult to obtain a perfect amorphous film because plasmas brought into contact with a film during deposition cause damage to the film and exert an annealing-like effect on the film to create microcrystals therein. Thus, any amorphous iron-silicide film exhibiting semiconductor characteristics close to those of β-FeSi₂ has not been obtained.

The inventor has found that an extremely high-quality FeSi₂ in the amorphous state can be obtained by depositing a non-granular flat or continuous film using a sputtering process capable of depositing high-energy particles, and the FeSi₂ in the amorphous state exhibits semiconductor characteristics close to β-FeSi₂.

Specifically, the present invention provides an amorphous iron-silicide film consisting of an amorphous FeSi₂ film having a bandgap of 0.6 to 1.0 eV and exhibiting semiconductor characteristics, which is obtained through a sputtering process.

The present invention also provides a method of preparing an amorphous FeSi₂ film having a bandgap of 0.6 to 1.0 eV and exhibiting semiconductor characteristics close to β-FeSi₂. The method comprises depositing FeSi₂ as a continuous film on a substrate maintained at a temperature of less than 400° C. through a sputtering process under an Ar gas pressure of 5 mTorr or less using a FeSi₂ alloy target having a Fe:Si atomic ratio of 1:2.

The amorphous FeSi₂ is obtained through a low-pressure sputtering under an Ar gas pressure of 5 mTorr or less. In particular, a facing-targets type sputtering process may be used to grow the amorphous FeSi₂ film with higher quality.

FIG. 1 is a conceptual diagram showing the principle of a facing-targets type DC sputtering process. In this process, a magnetic field B applied in parallel to an electric field acts to completely confine plasmas between a target 2 and a target 3, so that the plasmas is not brought into contact with a substrate 1 disposed in a direction perpendicular to the targets 2 and 3, and thereby only neutral particles are deposited on the substrate 1. Thus, a higher-quality amorphous film can be obtained without damage due to the plasmas and creation of microcrystals arising from the annealing-like effect. In addition, the surface of the deposited film having a low temperature rise allows a continuous film to be grown only through the deposition process.

The non-contact between the deposited film and the plasmas can also prevent the occurrence of re-sputtering to allow the composition of the obtained film to have an extremely small mismatching with that of the targets. Thus, as with a laser abrasion process, FeSi₂ alloy target can be used. Further, the sputtering process can be performed under a low pressure of 5 mTorr or less, preferably 1 mTorr or less, so that particles (atoms) emitted from the targets reach the substrate while maintaining a high energy, approximately without collision with the Ar gas as a sputtering gas. The above two improvements in the sputtering process of the present invention allows an amorphous iron-silicide film to be grown with a higher quality as comparted with a conventional RF magnetron sputtering process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram showing the principle of a facing-targets type DC sputtering process.

FIGS. 2(a) and 2(b) are photographs showing SEM images of the respective surfaces of an amorphous iron-silicide film prepared through a method of the present invention and a polycrystalline β-FeSi₂ film.

FIG. 3 is a graph showing a substrate-temperature dependence of an X-ray diffraction pattern of the iron-silicide film prepared through the method of the present invention.

FIG. 4 is a graph showing an optical absorption spectrum and a substrate-temperature dependence of an absorption coefficient α of the iron-silicide film prepared through the method of the present invention.

FIG. 5 is a graph showing an optical absorption spectrum and a substrate-temperature dependence of an optical bandgap Eg of the iron-silicide film prepared through the method of the present invention.

FIG. 6 is a graph showing respective substrate-temperature dependences of a sheet resistance and a specific resistance ρ of the iron-silicide film prepared through the method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

Iron-silicide thin films having a film thickness of about 240 nm were formed on a Si (100) substrate and a Si (111) substrate at temperatures in the range from a room temperature up to 400° C. through a sputtering process using a facing-targets type DC sputtering apparatus (Mirror Tron Sputtering System MTS-L2000-2T: available from THIN-FILM PROCESS SOFT INC.) For comparison, iron-silicide films were formed under the same conditions except that substrate temperatures were maintained at more than 400° C. A FeSi₂ alloy (99.99%) having a Fe:Si composition ratio of 1:2 was used as a target. A sputtering chamber was evacuated to 10⁻⁴ Pa or less using a turbo-molecular pump. Then, during film formation, an Ar gas was supplied to the sputtering chamber at a flow rate of 15 sccm to adjust an Ar gas pressure at 1.0 mTorr, and a voltage and current to be applied were set at 950 mV and 6.0 mA, respectively. A deposition rate was 1.0 nm/min.

Formed films were evaluated by the observation using a SEM (scanning electron microscope), and the measurements of X-ray diffraction, optical absorption spectrum and electrical resistance. Based on the X-ray diffraction measurement, it was proved that the film is formed in the amorphous state when the substrate temperature is 400° C. or less. Based on the optical absorption spectrum measurement, it was also proved that the amorphous FeSi₂ film has a bandgap of 0.6 to 0.7 eV.

FIG. 2 shows SEM images indicating the change in surface configuration of the iron-silicide film depending on the substrate temperature. Each of the samples had an extremely smooth surface irrespective of the substrate temperature. A wave-shaped irregularity was slightly observed in a sample formed at a substrate temperature of 800° C. FIG. 3 shows a substrate-temperature dependence of an X-ray diffraction pattern. As seen in FIG. 3, an amorphous FeSi₂ film is obtained at a substrate temperature of less than 400° C., and a β-FeSi₂ film is obtained at a substrate temperature of 400° C. or more.

FIG. 4 shows an optical absorption spectrum, and a substrate-temperature dependence of an absorption coefficient α. The amorphous FeSi₂ film has an absorption coefficient α of 1.3×10⁵ to 1.6×10⁵ cm⁻¹, and the polycrystalline β-FeSi₂ film has an absorption coefficient α of 5.0×10⁵ to 7.8×10⁵ cm⁻¹. FIG. 5 shows an optical absorption spectrum and a substrate-temperature dependence of an optical bandgap Eg. The amorphous FeSi₂ film has an optical bandgap Eg of 0.64 to 0.82 eV, and the polycrystalline β-FeSi₂ film has an optical bandgap Eg of 0.84 to 0.94 eV. FIG. 6 shows respective substrate-temperature dependences of a sheet resistance and a specific resistance ρ. The amorphous FeSi₂ film has a specific resistance ρ of 3.2×10⁻³ to 7.3×10⁻³ Ωcm, and the polycrystalline β-FeSi₂ film has a specific resistance ρ of 1.0×10⁻¹ to 3.2×10⁻¹ Ωcm.

INDUSTRIAL APPLICABILITY

As with the laser abrasion process, the facing-targets type sputtering process can readily achieve a characteristic improvement based on addition of another element, which is effective to an amorphous film.

While a conventional sputtering process can be improved such that plasmas are not brought into contact with a film during film formation to prevent an annealing-like effect from acting on the film, so as to obtain a high-quality amorphous film, the facing-targets type sputtering process capable of isolating plasmas from a substrate can readily provide an amorphous film. Thus, the facing-targets type sputtering process can be industrially applied to the production of a multilayer or large-area amorphous film without any difficulties.

In addition, the amorphous ion-silicide can be formed as a magnetic semiconductor by adding a magnetic element thereto, or can be hydrogenated to adjust the carrier concentration thereof. Further, the amorphous ion-silicide capable of growing at a room temperature can eliminate the need of providing a mechanism for heating a substrate.

The amorphous ion-silicide film exhibiting semiconductor characteristics is applicable to a solar cell element and a light-receiving element for communication systems. 

1. An amorphous FeSi₂ film having a bandgap of 0.6 to 1.0 eV and exhibiting semiconductor characteristics, which is obtained through a sputtering process using a FeSi₂ alloy target having a Fe:Si atomic ratio of 1:2.
 2. A method of preparing an amorphous FeSi₂ film having a bandgap of 0.6 to 1.0 eV and exhibiting semiconductor characteristics, said method comprising depositing FeSi₂ as a non-granular flat continuous film on a substrate maintained at a temperature of less than 400° C. through a sputtering process in a sputtering chamber evacuated to a pressure of 10⁻⁴ Pa and supplied with an Ar gas at a pressure of 5 mTorr or less using a FeSi₂ alloy target having a Fe:Si atomic ratio of 1:2.
 3. The method as defined in claim 2, wherein said sputtering process is a facing-targets type sputtering process. 